Altostratus lenticularis

I awoke this morning (Jan 23rd) to find that my weather station was recording the first sub-zero temperature (-0.3°C) since I acquired it last November. With nearly a cloudless sky and hoar frost on the ground I drove out to the end of Askam Pier so see if there was any snow on the hills. There was indeed (but not much) but conditions were good for generating altostratus lenticularis.

View from the end of my street - the clouds are altostratus lenticularis

Moreover, feeding in the creek beside the pier was a Spotted Redshank in winter plumage.

Spotted Redshank, an un -common relative of the 'ordinary' Redshank

Now, back to the clouds. Altostratus means "medium height" and "layered". Lenticularis means lens-like. These clouds are formed when moist air moves across a mountain range (the Cumbrian Fells, in this case) and is forced to rise until it reaches colder air above and condenses into cloud. Because of some atmospheric condition, typically a temperature inversion , the cloud does not rise higher to form cumulus but instead spreads out to form stratus (in this case altostratus, because of its height).

Because the air is moving, and also undulating over the mountains, the stratus doesn't form a continuous layer but instead breaks up into individual "lenses" which move away and the next one if formed.

Once the lenses have cleared the mountains, it is quite common for them to become "stacked".




Stacked lenticular clouds today over Walney Island

Ocasionally, the lenticularis can be dramatic, and not suprisingly they have been mistaken for UFOs! The image below is courtesy of the Cloud Appreciation Society.

What would you think if you saw this at night, back-lit by the moon?

Answers on a postcard...